Preheat the oven to 350 and prepare a cookie sheet with parchment. Set to the side.
In a medium-sized bowl, combine the flour, baking soda, salt, and cornstarch. Set to the side.
In a medium-sized bowl with a hand mixer or in the bowl of a stand mixer with a paddle attachment, cream the peanut butter, butter, and sugars. Cream the mixture for 2-3 minutes, until light and fluffy. Scrape down the bowl as needed. Add in the egg and vanilla, mixing well to combine.
Add the flour mixture to the butter mixture and mix fully to combine. Scrape down the bowl as needed.
To form the cookies, scoop out a ball of cookie dough, about 1-2 tablespoon in size. Use the back of the cookie scoop or your thumb to make an imprint on the cookies. This is where you will put the fluff. Place the fluff in the dent in the cookie and bring the cookie dough around the outside to cover it. The marshmallow fluff should be completely covered and inside of the cookie dough ball.
Bake the cookie dough balls for about 10-12 minutes in the oven. The cookies will be ready when the edges are firm and the fluff is fully spread out. Let the cookies cool on the rack for about 10 minutes and then transfer to a cooling rack.
